Think a return to the bad old days when New York was a cesspool of crime and grime is impossible?

Guess again.

As The Post’s Michael Gartland recently reported, Grand Central Terminal is once again being flooded by droves of homeless people. And police insist there’s little they can do about it.

They do periodically roust the vagrants. But they won’t forcibly remove them until 2 a.m., when the station closes for the night.

It’s ironic that this news should come just as Grand Central — having been restored to its former grandeur and reclaiming its rightful place as one of New York’s crown jewels — celebrates its 100th anniversary.

Before Ed Koch became mayor, the terminal was a symbol of hopelessness and despair. Even well into Koch’s tenure, Grand Central was basically the city’s largest homeless shelter, with some 500 people — many if not most of them mentally ill — living there at any given time.

Since then, a whole generation of New Yorkers has now grown up never having known that depressing era — nor learning the lesson that when public places become inhospitable to the general public, it has a mushrooming effect. Witness what happened to the surrounding neighborhoods when Occupy Wall Street took over Zuccotti Park.

Obviously, the city must do more to address the homeless problem than just evict them from public places. But let’s hope we don’t have to refight the obvious: You do neither the city nor the homeless any favors by allowing the latter to make their home in Grand Central Terminal.